maint: use consistent if-else braces in xen and friends
    
    I'm about to add a syntax check that enforces our documented
    HACKING style of always using matching {} on if-else statements.
    
    This patch focuses on code related to xen.
    
    * src/libxl/libxl_conf.c (libxlCapsInitGuests)
    (libxlMakeDomBuildInfo): Correct use of {}.
    * src/xen/xen_hypervisor.c (virXen_getvcpusinfo)
    (xenHypervisorMakeCapabilitiesInternal): Likewise.
    * src/xen/xend_internal.c (xenDaemonOpen)
    (xenDaemonDomainMigratePerform, xend_detect_config_version)
    (xenDaemonDetachDeviceFlags, xenDaemonDomainMigratePerform)
    (xenDaemonDomainBlockPeek): Likewise.
    * src/xenapi/xenapi_driver.c (xenapiConnectListDomains)
    (xenapiDomainLookupByUUID, xenapiDomainGetOSType): Likewise.
    * src/xenconfig/xen_common.c (xenParseCPUFeatures, xenFormatNet):
    Likewise.
    * src/xenconfig/xen_sxpr.c (xenParseSxpr, xenFormatSxprNet)
    (xenFormatSxpr): Likewise.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>