log: fix deadlock obtaining hostname (related CVE-2018-6764)

The fix for CVE-2018-6764 introduced a potential deadlock scenario
that gets triggered by the NSS module when virGetHostname() calls
getaddrinfo to resolve the hostname:

 #0  0x00007f6e714b57e7 in futex_wait
 #1  futex_wait_simple
 #2  __pthread_once_slow
 #3  0x00007f6e71d16e7d in virOnce
 #4  0x00007f6e71d0997c in virLogInitialize
 #5  0x00007f6e71d0a09a in virLogVMessage
 #6  0x00007f6e71d09ffd in virLogMessage
 #7  0x00007f6e71d0db22 in virObjectNew
 #8  0x00007f6e71d0dbf1 in virObjectLockableNew
 #9  0x00007f6e71d0d3e5 in virMacMapNew
 #10 0x00007f6e71cdc50a in findLease
 #11 0x00007f6e71cdcc56 in _nss_libvirt_gethostbyname4_r
 #12 0x00007f6e724631fc in gaih_inet
 #13 0x00007f6e72464697 in __GI_getaddrinfo
 #14 0x00007f6e71d19e81 in virGetHostnameImpl
 #15 0x00007f6e71d1a057 in virGetHostnameQuiet
 #16 0x00007f6e71d09936 in virLogOnceInit
 #17 0x00007f6e71d09952 in virLogOnce
 #18 0x00007f6e714b5829 in __pthread_once_slow
 #19 0x00007f6e71d16e7d in virOnce
 #20 0x00007f6e71d0997c in virLogInitialize
 #21 0x00007f6e71d0a09a in virLogVMessage
 #22 0x00007f6e71d09ffd in virLogMessage
 #23 0x00007f6e71d0db22 in virObjectNew
 #24 0x00007f6e71d0dbf1 in virObjectLockableNew
 #25 0x00007f6e71d0d3e5 in virMacMapNew
 #26 0x00007f6e71cdc50a in findLease
 #27 0x00007f6e71cdc839 in _nss_libvirt_gethostbyname3_r
 #28 0x00007f6e71cdc724 in _nss_libvirt_gethostbyname2_r
 #29 0x00007f6e7248f72f in __gethostbyname2_r
 #30 0x00007f6e7248f494 in gethostbyname2
 #31 0x000056348c30c36d in hosts_keys
 #32 0x000056348c30b7d2 in main

Fortunately the extra stuff virGetHostname does is totally irrelevant to
the needs of the logging code, so we can just inline a call to the
native hostname() syscall directly.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>